1. Field of the Invention
The present invention relates to a common mode choke coil. More specifically, the present invention relates to a structure of a common mode choke coil capable of providing improved characteristics at high frequencies.
2. Description of the Related Art
An alternating current (AC) power supply line constitutes a route through which external noises flow into an electronic apparatus or noises generated inside an electronic apparatus flow out. A choke coil is therefore inserted in series with the AC power supply line for cutting off the noises. There are two types of noises; a normal mode noise generated between lines and a common mode noise generated in both lines relating to the ground, and in general the common mode noise causes problems. It is required that a common mode choke coil for removing the common mode noise have a large inductance for the common mode noise and that magnetic fluxes cancel out each other for the AC voltage of a commercial power supply.
When a toroidal core is used as a magnetic core of the common mode choke coil, bifilar winding in which two wires are wound together generates little leakage flux. However, since insulation between the wires must be considered when a high voltage is applied therebetween and since also line capacitance between the wires must be considered in order to obtain preferable characteristics at high frequencies, the two wires must be wound apart from each other prohibiting the use of the bifilar winding. Accordingly, in general, one wire is wound around one half of the toroidal core, and another wire is wound around the other half thereof such that the both wires are wound equally so that respective magnetic fluxes cancel out each other relative to a normal mode current.
FIGS. 9 and 10 show conventional common mode choke coils. FIG. 9 is a perspective view of a conventional toroidal common mode choke coil of vertical type. FIG. 10 is a perspective view of a conventional toroidal common mode choke coil of horizontal type. In FIGS. 9 and 10, a plastic insulating partition 90 is arranged across a center opening of a plastic insulating case (hereinafter, referred to as a bobbin) 95 housing a toroidal core, and two coils 93 and 94 are wound in the same direction so as to sandwich the plastic insulating partition 90. The magnetic core is mounted vertically on a plastic base 91 in FIG. 9, and horizontally in FIG. 10. The two coils lead out from respective corresponding terminals 92a and 92b, and 92c and 92d (92d not shown) via respective corresponding through-holes 93b and 94b formed in the plastic base 91 and two other holes (not shown) formed in symmetric relation thereto in the base 91. Terminations of the coils 93 and 94 are connected to the respective corresponding terminals 92a and 92b, and 92c and 92d. To be specific, one termination of one coil wound around one half of the bobbin 95 is connected to the terminal 92a and the other termination thereof is connected to the terminal 92b. Similarly, one termination of the other coil wound around the other half of the bobbin 95 is connected to the terminal 92d (not shown) and the other termination thereof is connected to the terminal 92c. 
The coils are wound around the bobbin 95 which is shown in an exploded view in FIG. 11. More specifically, the bobbin 95 is configured in such a manner that a toroidal core 113 is housed in a cylinder 114 composed of an insulating material and having an opening 112 at the center thereof and a cover 110 is fitted thereto. One of the two coils is wound around one half of the bobbin 95 having a configuration described above and the other coil is wound around the other half thereof such that the both coils are wound equally so that respective magnetic fluxes cancel out each other relative to a normal mode current.
Typically, in the bobbin 95, a partition 90 is inserted separating the both coils in order to improve the withstand voltage therebetween. The cylinder 114 and the cover 110 have respective fitting mechanisms 115, and 115xe2x80x2 and 115xe2x80x3 for fitting the partition 90 thereto.
The vertical type toroidal common mode choke coil shown in FIG. 9, which can save an installation area, is generally used in a circuit board,.
FIG. 12 is a sectional view for explaining problems in the vertical type toroidal common mode choke coil. In FIG. 12, the bobbin 95 is mounted on the base 91 and the coil 93 is wound with a regular interval around one half of the bobbin 95. One termination of the coil 93 is connected to the terminal 92b, and the other termination is connected to the terminal 92a. Parts 12A and 12B of a toroidal core 12 are shown at a section of the bobbin 95. The coil 93 is shown only at its starting and finishing ends.
In such a configuration, the coil 93 starting from the terminal 92b provided on the base 91 located under the bobbin 95, that is, toward the part 12B of the core 12, is wound all the way around the half of the bobbin 95. The coil 93 then passes diagonally through the center opening of the bobbin 95 from upper part of the bobbin 95, that is, the part 12A, toward the terminal 92a provided on the base 91, when travelling from one end face of the bobbin 95 to the other end face thereof, and terminates there to be connected to the terminal 92a. 
In the aforesaid case, a starting end part A of the coil 93 is disposed in contact with or close to the bobbin 95, and a finishing end part C is disposed in contact with or close to a portion of the bobbin 95 where the coil 93 starts (hereinafter, referred to as beginning of the coil) in order to prevent both starting and finishing end parts of the coil 93 from getting damaged by a vibration or contact. However, in such an arrangement, the following problems arise. Referring to FIG. 13, the problems will be described.
FIG. 13 is an electrical equivalent circuit of the arrangement of FIG. 12. The coil 93 includes an input terminal XA and an output terminal XB, and the coil 94 includes an input terminal YA and an output terminal YB. Both the coils 93 and 94 are wound around the toroidal core 12, and have inductances ZLX and ZLY, respectively. The stray capacitance (hereinafter, referred to as a capacitance) of each part will be defined as follows.
Reference symbol CL1 denotes a capacitance between the inputs of the coils 93 and 94. Reference symbol CL2 denotes a capacitance between the outputs of the coils 93 and 94. Reference symbols C1X and C2X denote capacitances generated due to the finishing end part C of the coil 93 disposed close to or in contact with a beginning of the coil 93, as shown in FIG. 12. Similarly, reference symbols C1Y and C2Y denote capacitances generated due to the finishing end part C of the coil 94 disposed close to or in contact with a beginning of the coil 94.
Other symbols CS1, CS2, CS3, CS4, CS5, CS6 and CS7 denote line capacitances between the coils.
The common mode choke coil is inserted in series with an AC power supply line in order to cut off a noise. More specifically, the terminals XA and YA are used as input terminals and the terminals XB and YB are used as output terminals. When the terminals XA and YA are connected to each other to form a terminal XYA and the terminals XB and YB are connected to each other to form a terminal XYB, it is preferred that an impedance ZAB between the both connection terminals be large with respect to the common mode noise.
As is evident from FIG. 13, the impedance ZAB between the both connection terminals is not influenced by the capacitances CL1 and CL2, and becomes an impedance ZAB of the circuit, which is connected in parallel with the inductance ZLX of the coil 93 and the inductance ZLY of the coil 94, the capacitances C1X, C2X, C1Y and C2Y, and the capacitances CS1, CS2, CS3, CS4, CS5, CS6 and CS7. In such a circuit, the inductances ZLX and ZLY are dominant at low frequencies, and the capacitances C1X, C2X, C1Y and C2Y are dominant at high frequencies. The coils can be wound around the bobbin 95 such that the influence of the capacitances CS1, CS2, CS3, CS4, CS5, CS6 and CS7 can be ignored as a distributed constant circuit, compared with the capacitances C1X, C2X, C1Y and C2Y.
In other words, the impedance ZAB between the both connection terminals is decreased by the capacitances C1X, C2X, C1Y and C2Y at high frequencies, so that the performance of the common mode coke coil with respect to a high-frequency noise is lowered. Accordingly, it is desirable to decrease the capacitances C1X, C2X, C1Y and C2Y in order to improve the performance with respect to a high-frequency noise.
FIG. 8 shows an example of frequency characteristics of a toroidal common mode choke coil of vertical type. In FIG. 8, the results are plotted with frequency in abscissa and with the impedance ZAB between the both connection terminals in ordinate. It is found that when the frequency exceeds 10 MHz, the impedance ZAB between the both connection terminals decreases in the conventional toroidal common mode choke coil of vertical type, as shown by reference numeral 82. This is because the influence of the capacitances C1X, C2X, C1Y and C2Y appears in the vicinity of 10 MHz and upward. Consequently, by the conventional method, the performance with respect to the common mode noise of 10 MHz or more is lowered.
The capacitances C1X, C2X, C1Y and C2Y are generated because the finishing end part C of the coil 93 is disposed in contact with or close to the beginning of the coil 93, as shown in FIG. 12. It is found that the existence of such capacitances lowers the performance of the toroidal common mode choke coil of vertical type at high frequencies. Such a problem that the performance at high frequencies is lowered due to the capacitances generated because of the finishing end part C of the coil 93 staying in contact with or close to the beginning of the coil 93 can be easily avoided in the toroidal common mode choke coil of horizontal type. This is because the input coil and the output coil can be positioned so as to oppose each other as shown in FIG. 10. However, since the choke coil of the horizontal type occupies a substantial area of the circuit board, the parts installation density is decreased, thereby hindering the downsizing of the apparatus.
Accordingly, it is an object of the present invention to provide a common mode choke coil which solves the above problems and improves characteristics at high frequencies.
In order to achieve the above objects, according to the present invention, in a common mode choke coil, in which one coil is wound around one half of a toroidal core housed in a bobbin and another coil is wound around the other half thereof such that the both coils are wound equally so that respective magnetic fluxes cancel out each other relative to a normal mode current, and in which one end part of each of the coils wound around respective halves of the core leads out toward a portion of the bobbin where the other end part thereof is wound, the one end part is disposed so as not to stay close to the portion of the bobbin where the other end part is wound, thereby reducing capacitances C1X, C2X, C1Y and C2Y.
In the common mode choke coil, the one end part of the coil may pass through a center opening of the core in a non-diagonal manner when travelling from one end face of the bobbin to the other end face thereof.
In the common mode choke coil, a plate composed of an insulating material may be provided between the one end parts and the coils including the other end parts.
In the common mode choke coil, the plate may be provided with a plurality of notches each for fixedly holding the one end part of each of the coils.
In the common mode choke coil, the plate may be formed of a same insulating material as that of a base on which the common mode choke coil is mounted, and integrated with the base.
In the common mode choke coil, the plate and the base, which are integrated with each other, may be each provided with a plurality of notches each for fixedly holding the one end part of each of the coils.
In the common mode choke coil, the plate may be provided with a plurality of notches each for fixedly holding the one end part of each of the coils, and the base may be provided with a plurality of terminals.